1. Field of the Invention
The present invention relates to a fan filter unit of a semiconductor device manufacturing clean room, and more particularly, to a fan filter unit having a monitoring device which allows the operating state of the fan filter unit to be determined with the naked eye.
2. Discussion of the Related Art
Industrial hygiene greatly influences the quality and yield in manufacturing semiconductor devices. The processes of manufacturing semiconductor devices are therefore carried out in clean rooms of high purity. In the clean room, contaminants, such as dust, are eliminated by an air stream. The air stream ideally creates a downwardly directed laminar flow with minimal drift or vortices.
FIG. 1 illustrates a simple structure of the interior of the clean room. A production line 1 is provided with manufacturing equipment 2. Multiple fan filter units 3 are installed on the ceiling and a grating 4 is installed at the bottom of the production line 1. The air forced through fan filter units 3 is supplied to the production line 1 and flows downward. Foreign substances generated in production line 1 are exhausted through the grating 4 by the downward flow, thereby maintaining the inside of production line 1 at a high purity state. The air exhausted through grating 4 flows back over the ceiling and is recirculated by fan filter unit 3, thereby creating a high purity clean room.
FIG. 2 illustrates a fan filter unit for maintaining the high purity state of the clean room according to the prior art. The conventional fan filter unit 3 consists of a blower 11, motor 12, and filter 13 for eliminating contaminants, such as dust, from the air. Motor 12 is exclusively controlled by a local control unit 14 containing a microprocessor.
Depending upon the size of the clean room, hundreds to thousands of fan filter units may be installed, each having such a structure. Since each fan filter unit contains individual motors 12, the rate at which the drive motors stop due to a malfunction increases as time passes, or they may reach their service life expiration and require replacement. Since the internal pressure will exceed the external pressure when the motor of the fan filter unit stops in the clean room, the air in the clean room begins to flow backwards, resulting in an increase in the amount of particulate accumulated in the clean room, contrary to the basic concept of maintaining the clean room free of such contaminants. Consequently, products manufactured near the area described above are contaminated, thereby decreasing their quality and hence, the yield of satisfactory products.
According to a survey of such clean rooms which have operated for 4 to 6 years, 15 to 25% of the fan filter units stop because of some malfunction or other reason. The seriousness of such conditions cannot be easily detected using the present monitoring system. In order to determine the condition of the fan filter units, every fan filter unit must be checked individually using directional transistors, supersonic waves, or a three-dimensional wind velocity measuring instrument. An operator must check the velocities of the air streams flowing through the fan filter units to determine their operating states, which is a time-consuming process. Problems occurring after an inspection cannot be identified until the next inspection. Thus, the affected area is left at a low purity state making the clean room inefficient.